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Do we need to continue androgen deprivation therapy?

21 March 2015Prof. Dr. Axel Merseburger

Fortunately, a growing number of treatment options exist to treat metastatic castrate-resistant prostate cancer (mCRPC); however, with these newer options many questions about optimising treatment remain unanswered.

There is an on-going discussion on the value and need of androgen deprivation therapy (ADT) in this highly palliative stage of the disease. Should ADT be continued when abiraterone, enzalutamide, or chemotherapy are initiated? Are there downsides to ADT in this stage? Does it affect the health-related quality of life (HRQoL)? Those questions are worthy of scrutiny.

This article is based on today’s presentation which will focus on the evidence for maintaining ADT in CRPC patients, the statements provide basis for a discussion of the evidence and the rationale behind the recommendation if/that ADT should be continued in mCRPC.

The European Association of Urology (EAU) guideline clearly states that patients with metastatic castrateresistant prostate cancer (mCRPC) should indefinitely continue androgen deprivation therapy (ADT); this recommendation applies to metastatic CRPC (mCRPC) and non-metastatic CRPC (nmCRPC)1. Other guidelines, such as that from the American Urological Association (AUA)2 and the National Comprehensive Cancer Network (NCCN)3, likewise mention the need to maintain ADT when mCRPC develops.

As support, all randomised controlled trials of the many newer agents for mCRPC, including abiraterone, enzalutamide, sipuleucel-T, radium 223 and cabazitaxel4-10 all had continuation of ADT and mainstay of castration levels of testosterone (<50 ng/ dl) as an inclusion criterion. However, this does not prove the value of ADT in this setting. In metastatic disease, while hormonal treatment improves symptoms11, there is no conclusive prospective evidence that lowering testosterone levels improves life expectancy12. With the event of evaluating novel substances in the setting of non-metastatic CRPC, the value of combining ADT with tertiary hormonal manipulation (e.g. Enzalutamide, Abiraterone, ARN-509, etc.) has not been investigated.

Sparse evidence

The evidence for continuing ADT in metastatic prostate cancer is based on sparse literature. A single study demonstrated that androgen priming in a small group of patients (n=85) using chemotherapy regimens that are now outdated altered the prognosis13.

However, it is becoming clear that within the prostate and prostate tumour microenvironment androgen activity continues even when serum testosterone levels are suppressed by ADT14, and intracrine androgen synthesis is sufficient to activate androgen receptor target genes15. Adaptive alterations include alternative androgen synthesis pathways, and androgen receptor overexpression, mutation and splice variations16. Furthermore, many mechanisms that may confer castration-resistance still require, or are enhanced by, the presence of androgens or androgen receptor ligands. Together these observations suggest that treatment combinations that include ADT and suppress intracrine and systemic androgen contributions are required in CRPC.

The presentation will cover the evidence and rationale for continuing ADT in CRPC when other treatments are initiated, and aim to provide clear statements on this issue.

The rationale for ADT use in combination with abiraterone acetat

Abiraterone acetat selectively inhibits the enzyme 17 α-hydroxylase/C17, 20-lyase (CYP17) and thus inhibits androgen biosynthesis18. Abiraterone acetat also has direct activity on reducing the expression of the androgen receptor gene in tumour cells19. This underscores the need to target as many parts of the androgen receptor signalling pathway as possible and therefore providing a rationale for combining abiraterone with ADT. Remarkably, experimental evidence suggests that the testosterone suppression achieved by abiraterone monotherapy is not sustained in non-castrated men and is overcome by a subsequent two- to three-fold surge in luteinising hormone (LH) levels20. Although the pharmacokinetic study of O’Donnell et al20 assessed a small number of men, it does suggest a need to maintain castrate levels of testosterone with ADT when initiating abiraterone therapy.

This rationale has been used in the pivotal phase III trials of abiraterone. The efficacy of abiraterone in combination with prednisolone was demonstrated in two trials in patients with mCRPC as previously published4,5 (LoE: 1b). Importantly, castration levels of testosterone were maintained in both these studies with the continuation of ADT.

Currently, a German multicentre trial (SPARE trial) is investigating the impact of continuing ADT when initiating abiraterone. This study (German Association of Urological Oncology trial number AUO 67/11) is investigating abiraterone monotherapy (plus prednisolone) versus abiraterone plus ADT (plus prednisolone) in 70 men with chemotherapy-naïve mCRPC. Preliminary results of this study should be available in 2016, which will provide the first prospective insight on the potential efficacy advantages of maintaining ADT when abiraterone treatment is initiated in mCRPC. Interestingly, there is no published data assessing abiraterone (with or without ADT) in patients with non-metastatic CRPC (nmCRPC). More data on the use of newer agents for the treatment of nmCRPC are needed.

The rationale for ADT use with enzalutamide

Androgen receptor signalling persists during castration, and several mechanisms, even in individual patients (through clonal heterogeneity), may explain this persistence23. Addition of androgen receptor blockers to ADT may therefore help achieve more complete androgen blockade. With the development of the novel androgen receptor blocker enzalutamide, which binds to the androgen receptor with eight-fold higher affinity than bicalutamide32, it is important to ask whether there is any clinical need to combine enzalutamide treatment with ADT.

To date, however, there have been no clinical trials comparing enzalutamide monotherapy with enzalutamide plus ADT to confirm the need for continued ADT when initiating abiraterone therapy in patients with mCRPC. Unlike bicalutamide, enzalutamide has no known agonist activity, and it is thought that bicalutamide resistance does not exclude subsequent enzalutamide use32. In addition, a recent report has assessed enzalutamide monotherapy in hormone-naive men with prostate cancer33. This initial report of enzalutamide monotherapy in 67 patients suggested a lower frequency of gynaecomastia (36%) than previously reported with bicalutamide monotherapy. Furthermore, PSA declines were of a similar magnitude to those achieved by ADT but adverse events were frequent and testosterone levels increased33.

Therefore, more data are needed to determine whether the combination of enzalutamide with ADT has a favourable efficacy and safety profile for the treatment of CRPC compared with enzalutamide monotherapy. In the meantime, as with abiraterone, pivotal trials of enzalutamide in men with CRPC included the need for castration maintenance with ADT6,7; these studies have shown that this combination improved OS when used before chemotherapy and after chemotherapy.

Ongoing clinical studies may provide some clues to the need for backbone ADT when initiating enzalutamide in patients with CRPC. However, there are no trials with enzalutamide that are equivalent to the SPARE study with abiraterone, which directly compare enzalutamide monotherapy with enzalutamide combined with ADT in men with CRPC.

ADT during chemotherapy

EAU guidelines state that ADT with GnRH analogues should be continued when giving mCRPC patients chemotherapy1. The theory for continuing ADT when starting chemotherapy in mCRPC is that pausing ADT may lead to renewed release of testosterone and stimulation of the remaining androgen-sensitive elements of the tumour. Conversely, approximately 50% of men receiving ADT in the long-term remain castrated for 2.5 years after stopping ADT34, and stopping ADT might re-induce hormone sensitivity35.

These conflicting viewpoints are difficult to prove as there is a lack of well-designed prospective trials exploring this issue, and retrospective data are conflicting36-38. There is no evidence that the combination of ADT with chemotherapy causes harm; however, there is also no strong evidence of benefit in this clinical scenario. Two ongoing studies (ClinicalTrials.gov identifiers: NCT01487902 and NCT01224405) are investigating the possible advantage of maintaining ADT during chemotherapy. One of these studies (NCT01487902) that is comparing docetaxel plus prednisolone with docetaxel plus prednisolone and leuprolide in approximately 90 men with CRPC was due for completion in October 2013, and results may therefore be available soon. Until these results are available ADT should be continued when chemotherapy is initiated in mCRPC in daily practice also according to the current guidelines.

Prospective data and biomarkers needed

All clinical trials of newer agents (and recent trials of chemotherapy agents) in mCRPC include patients who maintain castrate levels of testosterone, and so clinical practice should adhere to this principle of continuing ADT when initiating abiraterone, enzalutamide or chemotherapy. However, not only are more prospective data needed to assess the importance of backbone ADT in CRPC, but also reliable prognostic and predictive biomarkers are urgently needed to individualise treatment with newer agents, their combination with ADT, and the optimum treatment sequences.